builtins: accept old-style iterables to iter (#7817)

Author: hauntsaninja

stdlib/builtins.pyi

@@ -1202,9 +1202,15 @@ def help(request: object = ...) -> None: ...
 def hex(__number: int | SupportsIndex) -> str: ...
 def id(__obj: object) -> int: ...
 def input(__prompt: object = ...) -> str: ...
+
+class _GetItemIterable(Protocol[_T_co]):
+    def __getitem__(self, __i: int) -> _T_co: ...
+
 @overload
 def iter(__iterable: SupportsIter[_SupportsNextT]) -> _SupportsNextT: ...
 @overload
+def iter(__iterable: _GetItemIterable[_T]) -> Iterator[_T]: ...
+@overload
 def iter(__function: Callable[[], _T | None], __sentinel: None) -> Iterator[_T]: ...
 @overload
 def iter(__function: Callable[[], _T], __sentinel: object) -> Iterator[_T]: ...

test_cases/stdlib/builtins/test_iteration.py

@@ -0,0 +1,14 @@
+from typing import Iterator
+from typing_extensions import assert_type
+
+
+class OldStyleIter:
+    def __getitem__(self, index: int) -> str:
+        return str(index)
+
+
+for x in iter(OldStyleIter()):
+    assert_type(x, str)
+
+assert_type(iter(OldStyleIter()), Iterator[str])
+assert_type(next(iter(OldStyleIter())), str)